Abstract:

BACKGROUND : Ticks secrete a diverse mixture of secretory proteins into the host to evade its immune response and
facilitate blood-feeding, making secretory proteins attractive targets for the production of recombinant anti-tick
vaccines. The largely neglected tick species, Rhipicephalus zambeziensis, is an efficient vector of Theileria parva in
southern Africa but its available sequence information is limited. Next generation sequencing has advanced
sequence availability for ticks in recent years and has assisted the characterisation of secretory proteins. This study
focused on the de novo assembly and annotation of the salivary gland transcriptome of R. zambeziensis and the
temporal expression of secretory protein transcripts in female and male ticks, before the onset of feeding and
during early and late feeding.
RESULTS : The sialotranscriptome of R. zambeziensis yielded 23,631 transcripts from which 13,584 non-redundant
proteins were predicted. Eighty-six percent of these contained a predicted start and stop codon and were
estimated to be putatively full-length proteins. A fifth (2569) of the predicted proteins were annotated as putative
secretory proteins and explained 52% of the expression in the transcriptome. Expression analyses revealed that
2832 transcripts were differentially expressed among feeding time points and 1209 between the tick sexes. The
expression analyses further indicated that 57% of the annotated secretory protein transcripts were differentially
expressed. Dynamic expression profiles of secretory protein transcripts were observed during feeding of female
ticks. Whereby a number of transcripts were upregulated during early feeding, presumably for feeding site
establishment and then during late feeding, 52% of these were downregulated, indicating that transcripts were
required at specific feeding stages. This suggested that secretory proteins are under stringent transcriptional
regulation that fine-tunes their expression in salivary glands during feeding. No open reading frames were
predicted for 7947 transcripts. This class represented 17% of the differentially expressed transcripts, suggesting a
potential transcriptional regulatory function of long non-coding RNA in tick blood-feeding.
COCLUSIONS : The assembled sialotranscriptome greatly expands the sequence availability of R. zambeziensis, assists
in our understanding of the transcription of secretory proteins during blood-feeding and will be a valuable resource
for future vaccine candidate selection.